Field
This technology as disclosed herein relates generally to intra-laminar failures in composite materials and, more particularly, to reducing raw test data in lamina level composite failure mode testing.
Background
Matrix laminated composites are being applied increasingly in structures due to their high stiffness-to-weight and strength-to-weight ratios. However, they can develop internal damage, like matrix cracking and delamination, which degrades structural integrity leading to failure. The internal damage is not easily detectable. Transverse matrix cracking and delamination are a typical damage mechanism of composites especially when structures are submitted to bending loads.
There are two different failure mechanisms in composite failure. One is so called the intra-laminar failure in which the failure is happening within the laminar. The other is the inter-laminar failure where the delamination failure between layers. There is strong interaction between intra-laminar failure and inter-laminar failure. For example, the matrix crack initiated with a ply will lead to delamination at adjacent ply interface when the crack progress across plies. The measurement of both intra-laminar fracture toughness and inter-laminar fracture toughness are important. Since the matured ASTM test standard and associated test data reduction method for the inter-laminar fracture toughness is already exist, following discussion is focused on intra-laminar fracture.
The intra-laminar fracture characterization involves testing to produce two different type of fracture properties, namely fibre fracture property, and matrix fracture property. The fibre fracture is commonly characterized by a crack apparently running perpendicular to the fibre direction to break the fibre and is considered as direct cause leading to the composite ultimate failure. The matrix fracture, on the other hand, is characterized by a crack that is apparently running in parallel to fibres through the layers thickness and initiates the composite failure process. Both fracture properties are the fundamental material properties and are crucial for composite failure analysis.
The analysis that predicts the intra-laminar failure in composite laminate panels using energy-based evolution approach needs the laminar level of the strain energy release rate value input. This strain energy release rate value is a part of the material strength/energy data that is extracted from the certain type of material testing. Most of today's material data testing are conducted following national/industrial test standard. However, there is no test standard that exists for this type of testing for composites. Many researches performed in this field agree that the compact tension testing created originally for a metal material specimen can be used to extract the strain energy release rate for composite materials using different test data reduction methods. However, these data reduction methods either not reliable (significant scatter results), not compatible, or not applicable. The current compliance data reduction method has two major problems: 1) it fit the compliance curve individually with a number of the repeat test data and inherit the test data variability issue (this increases the variability of reduced results); 2) it fit the compliance curve, but results calculated from a formula involves the derivative of the compliance. Significant error is induced from taking derivative of the compliance. A better technique is needed to address the data scatter issue.